Radial compressor having improved press-fit pistons

ABSTRACT

A high speed radial compressor adapted for use in automobile air conditioning has pistons which are retained by a press-fit in a driving yoke. The pistons and/or yoke are immersed in a sodium stannate chemical immersion plating solution for a sufficient period to form a tin plate thickness of roughly 0.00005 inch. The tin plating provides the lubricant for a press-fit of unquestionable integrity characterized by high pull-apart resistance during compressor operation by facilitating the insertion of piston into the yoke without galling or deforming. A less preferred plating is cadmium.

United States Patent Henkel June 3, 1975 [54] RADIAL COMPRESSOR HAVING 3,149,543 9/1964 Naab 92 155 IMPROVE P'STONS 3133339, 53322 $22222? 133/533 Inventor: Barry Henkel, y Ohio 3:212:411 lO/l965 StormsIII::......,........:::::.. 92/248 [73] Assignee: General Motors Corporation,

Detroit, Mich. Primary ExaminerWil1iam L. Freeh Assistant Examiner-Gregory Paul LaPointe [22] Flled. June 28, 19 Armmey, Agent, or Firm--Frederick M. Ritchie [211 App]. No.: 483,963

Related US. Application Data [57] ABSTRACT [63] gg gggigrgg of M6932 A high speed radial compressor adapted for use in automobile air conditioning has pistons which are re- [52] U S Cl 417/53, 29/1564 92/172 tained by a press-fit in a driving yoke. The pistons 417/273 and/or yoke are immersed in a sodium stannate chem- [5 H Int Cl F04) 1/04 B23 15/00 ical immersion plating solution for a sufficient period [581 Fie'ld "3 495f92/15p5 72 to form a tin plate thickness of roughly 0.00005 inch. 92/255 156 4 74750 6 f The tin plating provides the lubricant for a press-fit of unquestionable integrity characterized by high pullapart resistance during compressor operation by facili- [56] References cued tating the insertion of piston into the yoke without galling or deforming. A less preferred plating is cad- UNITED STATES PATENTS mium. 2,598,975 6/1952 Coulter 29/525 2,710,l37 6/1955 Arnouil 92/172 9 Claims, 9 Drawing Figures RADIAL COMPRESSOR HAVING IMPROVED PRESS-F IT PISTONS This is a continuation-in-part of my application Ser. No. 416,932, filed Nov. 19, 1973, now abandoned.

This invention relates to a modified scotch yoke compressor for automotive air conditioning. More particularly, it concerns an improved press-fit providing a stable connection between compressor piston and its actuating yoke; and generally it provides a press-fit connection of unquestionable integrity by plating parts to be press-fit with tin or cadmium.

The problem in making the piston-yoke assembly for such compressor had been one of galling during the press, resulting in a press-fit of questionable integrity. A study of oil type lubricants resulted in the general conclusion ofeasy in easy out," i.e., the insertion of the parts was facilitated but their assembled life under cyclical axial and cocking load turned out to be unsatisfactory. Further, it has been found that tin plating, in addition to an existing interference relation of the parts, permits a press-fit at push-in speeds from /2 inch per minute to 20 inches per minute without galling in which 1) higher push-in loads can be developed upon insertion of piston in yoke, (2) pull-out loads required to disassemble the piston from the yoke are increased as compared to such parts with no plating, and (3) there is sustained resistance to cyclical pull-apart forces acting on the assembly.

I theorize that heat is generated at the asperities or high points of the mating parts as the parts are pressed together. This heat may tend to melt the tin at the asperities giving momentary lubrication to eliminate galling and provide for smooth insertion. The tin plating solution used is a conventional sodium stannate chemical immersion plating solution used heretofore on automotive pistons. The thickness of tin is calculated to be roughly 0.00005 inch and at least 0.00004 inch. Note that insofar as anti-galling is concerned other soft metals such as lead and cadmium should perform in the same manner as tin. Tests have confirmed this antigalling characteristic for each metal, but, surprisingly, pull-out load is not improved with lead whereas the increase in pull-out load with cadmium approaches that of tin.

Tests have also shown that a high final push-in force (regardless of push-in speed) will deform yoke at the piston shoulder and result in a press-fit of questionable integrity by loosening the piston. Therefore a harmful high final inertial loading between piston and yoke, especially on the piston shoulder, is to be avoided. As an example, in the piston-yoke assembly disclosed herein the final inertial load should be controlled to a maximum of 2,500 pounds to avoid yoke deformation.

As mentioned above, prior art pistons in automobile engines have been tin plated. However, such pistons have been tinned as an anti-scuffing surface to achieve a satisfactory break-in of the engine, not for press-fit devoid of galling. The prior art discloses a coating of cadmium, but the coating disclosed is relatively thick, mainly to fill voids between mating parts, and is believed to serve as an anticorrosion device to prevent the mating parts from freezing together, thereby achieving easy separation of the parts.

Accordingly, it is a general object of this invention to make a press-fit without galling and resistant to cyclical pull-apart force by plating one or both of the parts to be pressed together with tin or cadmium.

A more particular object is the provision in the manufacture of a modified scotch yoke compressor of a soft metal plating method for press-fitting pistons and yokes together.

Another object within the provision of the last object is such a'method which includes tin-plating the pistons as a lubricant for press-fit.

It is also an object to provide a radial compressor in which a press-fit piston-yoke assembly is actuated by a slider block, and in which the method of tin plating the yoke functions on the cavity side to prevent galling when press-fitting piston and yoke together and on the opposite surface as a lubricant for the actuation of the yoke by the slider block.

Another object of this invention is an improved method of pressfitting two aluminum parts together to form a stable connection suitable for reciprocation at high speeds and comprising the steps of forming a socket on one of the parts, forming a stem on the other of the parts to have an interference dimension with respect to the socket, chemically immersion plating either said stem or said socket in an immersion plating solution of tin or cadmium to a thickness of at least 0.00004 inch, and pressing said stem into said socket without galling and without deforming the parts by the final inertial load of the pressing thereby to form said stable connection.

A further object of this invention where higher pullout loads and better bearing characteristics are desired is an improved method of press-fitting an aluminum piston-yoke assembly to be driven by a slider block actuating mechanism and comprising the steps of forming a socket on the yoke, forming a stem on the piston to have an interference dimension with respect to the socket, immersion plating said yoke in a sodium stannate chemical immersion plating solution to a tin thickness of at least 0.00004 inch, and pressing said stem into said socket without galling or deforming to form a stable connection in said piston-yoke assembly and lubricity between said yoke and said slider block.

Still another object of this invention is an improved method of press-fitting an aluminum piston-yoke assembly to a driven by a slider block actuating mechanism and subject to cyclical reciprocating forces along an axis and concurrent lateral cocking forces of like frequency and comprising the steps of forming a socket on the yoke, forming a stem on the piston to have an interference dimension with respect to the socket, immersion plating said piston or yoke or both said piston and yoke in a sodium stannate chemical immersion plating solution to a tin thickness of at least 0.00004 inch, and pressing said stem into said socket without galling or deforming to form a stable connection in said piston-yoke assembly.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the Drawings:

FIG. 1 is a view in vertical section through a radial compressor incorporating the present invention;

FIG. 2 is a sectional view of a compressor piston in corporating the present invention;

FIG. 3 is a fragmentary sectional view of the stem portion of the piston in FIG. 2;

FIG. 4 is an elevational view partly in section of an actuating yoke incorporating the present invention and suitable for use in the actuating mechanism of FIG. 1 and taken with the view toward the inside of the yoke;

FIG. 5 is an elevational view of the actuating yoke with the view taken toward the back side of the yoke;

FIG. 6 is an elevational view of the yoke taken along line 6-6 in FIG. 4;

FIG. 7 is a side elevational view of the yoke with the yoke socket in section and taken along line 7-7 in FIG. 4;

FIG. 8 is an enlarged fragmentary sectional view of the socket portion of the yoke; and

FIG. 9 is a prospective view of an acutating yoke suitable for use with the present invention.

Referring now to the drawings, a radial compressor utilizing the subject invention is shown in FIG. 1. Basically, the subject invention is a modification of the compressor assembly described in US. application Ser. No. 275,553 filed July 27, 1972, now abandoned, and its copending continuation-in-part US. application Ser. No. 416,933 filed Nov. [9, l973. both applications being assigned to the assignee of this invention. The disclosures of those applications are incorporated by reference into the description of this invention.

The subject compressor includes a rigid cast aluminum cylinder housing 12 closed by an aluminum cylinder head 13, having oppositely extending cross bores 14 on a first axis of the housing and a second pair of cross bores (not shown) on another axis of the housing normal to the first axis as best shown in the aforementioned applications. Each of the cross bores terminate in a diametrically enlarged bore or counterbore 22 connecting with the aforementioned bores at a flange or shoulder 24. A valve plate 26 is positioned against the flange 24 and is retained by a snap ring 28 received in a groove 30 in the enlarged bore 22. An annular discharge reed plate 32 controls flow through a series of circumferentially spaced discharge apertures 34 and is retained by a plate 36, the valve plate assembly 37 or discharge valve means being secured together by a rivet 38 in a central aperture 40 in the valve plate 26. The reed plate 32 controls flow of pressurized gas into a discharge chamber 42. The cylinder housing is circular and rather drum-shaped in form and is enclosed on its outer periphery by a cylindrical steel band 44 enclosing the discharge chamber 42. The band 44 may be retained by a plurality of pins 45 in housing 12. Annular seals or O-rings 44a are received in housing grooves 44b thereby providing a fluid tight seal for discharge chamber 42.

The compressor is driven from the threaded end of a shaft 46 which includes spaced enlarged bearing portions 48 supported for rotation in housing 12 by main bearing assemblies 50. These bearing assemblies 50 are in communication with a central suction space or inlet chamber 100 via lubricant bleed passageways such as 39. The shaft 46 also includes an eccentric portion 52 having a slider block 54 mounted thereon for relative rotation between the shaft and the block via a plurality of separate elongated needle bearings 56. The needles 56 are retained against axial movement relative to the eccentric portion 54 by needle retainers 41. Sliders block 54 includes bearing surfaces 58 which engage and slide relative to bearing surfaces 62 of yoke 66.

The shaft 46 also supports a pair of counterweights 47 and engages a fluid seal assembly 49 as illustrated in FIG. 1.

The cylindrical housing 12 and cylinder head 13 each include inwardly extending annular flanges or cups 65 which receive the previously described shaft portions 48 and their associated bearing assemblies 50. The counterweights 47 are mounted to shaft eccentric 52 by a rivet 47a extending through an aperture 47b in the eccentric. A thrust washer 67 is provided between annular flange 65 on housing 12 and its associated counterweight 47. A thrust bearing assembly 69 is positioned between annular flange 65 on the cylinder head 13 and its associated counterweight 47.

A piston-yoke assembly 60 formed in accordance with the press-fit method of this invention includes the yoke 66 and two piston assemblies 74 extending from opposite ends of the yoke. The yoke 66 is machined to include cavities or sockets 70 each receiving a reduced stem portion 72 of a piston assembly 74. The stem por tion 72 is press-fitted within the cavity 70 to form a rigid connection between the yoke and the piston assembly. The piston assembly 74 can be formed to include a shoulder 76 engaging outer surface 78 of the yoke to aid in the stability of the connection. Piston assembly 74 comprises a diametrically enlarged central portion 80 which terminates in an integral cup portion 82 having a relatively short skirt portion 84 providing an outer surface 86 engaging cylinder wall 88 of the cylindrical housing 12. The outer surface 86 of the piston assembly 74 contains a relatively wide groove 90 receiving a matched Teflon sealing ring 92 providing a fluid tight seal between outer surface 86 of the piston and the cylinder wall 88. The piston assembly also includes circumferentially spaced inlet apertures 94 which are normally closed by a suction reed plate 96 or suction valve means which is operative to regulate flow of gas into the compression chamber 98 from an inlet chamber 100.

In operation, rotation of shaft 46 results in reciprocating movement of the slider block 54 along two axes which are normal to one another so as to provide reciprocation of the piston-yoke assembly 60 within its respective cross bores. Such reciprocation can reach extremely high speeds such as when the compressor shaft 46 is driven at speeds upt 9,000 revolutions per minute. At high speeds, considerable stress is placed on the pressfit connection between piston and yoke. This invention insures the integrity of the connection throughout the operating life of the compressor.

During compressor operation, piston assembly 74 moves within cross bore 14 toward the center of the cylindrical housing 12, causing the gas to open the suction reed plate 96 forcing refrigerant from inlet chamber 100 through apertures 94 into compression chamber 98 while at the other end of yoke 66 the other piston assembly 74 is being extended to compress refrigerant. Then movement of piston assembly 74 radially outwardly in bore 14 compresses the refrigerant in chamber 98 opening discharge reed plate 32 so that the pressurized refrigerant flows through apertures 34 into discharge chamber 42 for supply to the vehicle air conditioning system.

Stem 72 of each piston assembly 74 is received within the respective cavity or socket 70 in each end of the yoke 66. The press-fit connection between the yoke and each piston provides stable connections in the piston-yoke assembly 60. Such stable connections allow use of a relatively short skirt portion 84 on the piston since the piston skirt is not needed as a guide. Also, provision of a relatively wide groove 90 in the outer surface 86 of the short skirt 84 with a Teflon sealing ring 92 therein results in a relatively wide bearing surface between the piston skirt and the cylinder walls 88. This accomplishes a fluid tight seal while providing substantial area of engagement preventing metal-to-metal contact between the piston skirt 84 and the cylinder walls 88. Consequently, the piston can be substantially reduced in size and can be readily assembled into the yoke by virtue of the press-fit connection.

A suitable press-fit in a preferred embodiment avoids galling either the stem 72 or socket 70 or cracking or otherwise deforming the yoke 66, especially by the final inertial loading of the shoulder 76 thereon, and may be accomplished by providing predetermined interference dimensions and configurations for the yoke socket 70 and its mating piston stern portion 72. In an aluminum die cast yoke, for instance, socket 70 of the yoke may have an inside diameter Z between 0.4985 inch and 0.4995 inch while the piston stem portion of an aluminum die cast piston may have a finished outside diameter A between 0.5015 inch and 0.5020 inch after plating, i.e., an interference between the parts of 0.0020 inch to 0.0035 inch. Press-fit is facilitated by providing a lead-in portion 170 on the yoke socket and tween. A hollow 79 in piston stem 72 provides a reservoir or compression chamber to store air trapped between piston stem and yoke socket during press-fitting. Proper alignment of piston stem and yoke socket before press-fitting is also important to facilitate rapid assembly of piston and yoke. For this purpose, end 173 of the stem portion 72 and end 171 of the yoke socket have a bevel of chamfer Y of 45 for initial align ment clearance. Alignment is also important in achieving the desired sealing and bearing characteristics between the piston ring and cylinder bore. For this purpose, the surface of piston stem 72 should have a squareness of 0.0003 inch with respect to the face surface 81 of the piston.

The press-fit of this invention is improved in my preferred embodiment by tinning the yoke. In general, the yoke is immersed in a sodium stannate chemical immersion plating solution for a sufficient period to form a tin plate thickness of substantially 0.00005 inch and at least 0.00004 inch. The tin plating provides the lubricant for a press-fit of unquestionable integrity during compressor operation by facilitating the insertion of piston into the yoke without galling or deforming.

in particular, the following process specification may be followed for tin plating aluminum yokes to be used in the press-fit of this invention. As a general rule, it is well to avoid any contamination on the yoke which cannot be removed by the clean step below:

MAKE-UP AND CONTROL OPERATION TEMPERA- TURE SEQUENCE TlME IN BATH OF BATH SOLUTIONS AND RlNSES a. Clean 3 to 5 160 F. to Cleaner solution Minutes 180 F. which is inhibited to prevent attack on aluminum.

b. Rinse (Thoroughly) F. to Clean tap water.

c. Nitric (Dip) Room 20% minimum of Acid Temperature nitric acid.

d. Rinse (Thoroughly) Cold Clean tap water.

e. immersion 40 to 50 F. Dissolve 6 ounces Tin seconds 10 F. per gallon of Plate sodium stannate.

Maintain the concentration to 2.5 3.0 ounce tin plate per gallon by adding sodium stannate.

f. Rinse 30 seconds Cold Thoroughly rinse (minimum) in clean running water.

g. Rinse 10 seconds Cold Thoroughly rinse in clean water.

h. Dry 250 F. Dry until all to moisture retained 300 F. in the part is (Part Temp.) removed.

172 on the piston stem having a bevel or chamfer X of 10. When piston and yoke are pressed together, the resultant interference of such dimensions over a length C of 0.320 inch provides a stable connection therebe- The yoke is now ready for incorporation in piston assembly 74. The piston assembly is then ready to be press-fit assembled with yoke 66 by inserting assembly 74 through the cross bores of the cylinder housing as taught in the aforementioned copending application Ser, No. M6933.

investigation has also shown that a heavier tin plate than that obtained with the above 40-second immersion plating (eg. 4 minutes) increases the pull-out strength as does plating both piston and yoke While the embodiment of the present invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is:

l. A radial compressor of the type including a cast cylinder housing having cross bores including cylinder walls therein along axes normal to each other, said housing having one closed end and a removable cylinder head closing its other end; a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, a pair of yokes disposed normal to each other and having bearing surfaces in engagement with normal surfaces of said slider block, pairs of piston assemblies connected to each yoke so that the pistons extend oppositely from each other from said yokes, said piston assemblies being press-fit connected to said yokes providing a stable connection therebetween, said press-fit connection comprising an aluminum stem on each of said piston assemblies and an aluminum socket on its respective yoke, the diameter of said stem before the press-fit connection being larger than the diameter of said socket and one of said stem or said socket having a thin surface plating of soft metal having the characteristic to form a lubricant as the stem is pressed into the socket thereby to facilitate the press-fit connection, said pistons including a pistonskirt having a sealing ring disposed around the skirt providing a bearing surface between said piston skirt and the cylinder walls of said cross bores, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providing substantial area of engagement preventing metal-to-metal contact between said piston skirts and said cylinder walls, and valve means to direct fluids to and from the cross bores.

2. The radial Compressor of claim 1 wherein the soft metal is taken from the group consisting of tin and cadmium and the thin plating is at least 0.00004 inch thick.

3. A radial compressor of the type including a cast cylinder housing having cross bores including cylinder walls therein along axes normal to each other, said housing having one closed end and a removable cylinder head closing its other end; a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, a pair of aluminum yokes disposed normal to each other and having bearing surfaces in engagement with normal surfaces of said slider block, pairs of piston assemblies connected to each yoke so that aluminum pistons extend oppositely from each other from said yokes, said piston assemblies being press-fit connected to said yokes providing a stable connection therebetween of unquestionable integrity and characterized by high pull-apart resistance during compressor operation, said press-fit connection comprising a stem on each of said pistons and a socket on its respective yoke, the diameter of said stem before the press-fit connection being large than the diameter of said socket and said stem having a thin plating taken from the group consisting of tin and cadmium to form a gall-preventing lubricant for a press-fit connection without deformation of said socket, said pistons including a piston skirt including an annular groove therein extending a substantial width along said skirt, a sealing ring disposed in said groove providing a bearing surface between said piston skirt and the cylinder walls of said cross bores, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providing substantial area of engagement preventing metal-tometal contact between said piston skirts and said cylinder walls. and valve means to direct fluids to and from the cross bores.

4. A radial compressor of the type including a cast cylinder housing having cross bores including cylinder walls therein along axes normal to each other. said housing having one closed end and a removable cylinder head closing its other end; a drive shaft rotatably mounted in said housing and said cylinder head, an cecentric on said drive shaft, a slider block rotatably mounted on said eccentric, a pair of aluminum yokes disposed normal to each other and having bearing surfaces in engagement with normal surfaces of said slider block, pairs of piston assemblies connected to each yoke so that aluminum pistons extend oppositely from each other from said yokes, said piston assemblies being press-fit connected to said yokes providing a stable connection therebetween of unquestionable integrity and characterized by high pull-apart resistance during compressor operation, said press-fit connection comprising a stem on each of said pistons and a socket on its respective yoke, the diameter of said stem before the press-flt connection being larger than the diameter of said socket and the socket and bearing surfaces of said respective yoke having a thin plating taken from the group consisting of tin and cadmium as a gallpreventing lubricant for a press-fit connection without deformation and for the engagement of the bearing surfaces with said slider block, said pistons including a piston skirt including an annular groove therein extending a substantial width along said skirt, a sealing ring disposed in said groove providing a bearing surface be tween said piston skirt and the cylinder walls of said cross bores, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providing substantial area of engagement preventing metal-to-metal contact between said piston skirts and said cylinder walls, and valve means to direct fluids to and from the cross bores.

5. A method of press-fitting an aluminum compressor piston to an aluminum compressor yoke to form a stable connection suitable for reciprocation at high speeds along an axis and subject to concurrent lateral cocking forces of like frequency and comprising the steps of forming a socket on the aluminum yoke, forming a stem on the aluminum piston with an interference dimension with respect to the socket, immersion plating one of said stern and said socket in a sodium stannate chemical immersion plating solution to a tin thickness of substantially at least 0.00004 inch; and pressing said stem into said socket wihtout galling either said stem or said socket while controlling the press of said stem with respect to said yoke to an amount at which the final inertial load between said stem and said yoke will not cause deformation of said socket, thereby to form said stable connection.

6. A method of press-fitting two aluminum parts together to form a stable connection of unquestionable integrity and characterized by high pull-apart resistance suitable for operation at high speeds and comprising the steps of forming a socket on one of the parts, forming a stem on the other of the parts with an interference dimension with respect to the socket, immersion plating said socket in a sodium stannate chemical immersion plating solution to a tin thickness of substantially at least 0.00004 inch; and pressing said stern into said socket without galling either said stem or said socket while controlling the press of said stem with respect to said socket to an amount at which the final inertial load between said stem and said socket will not cause deformation of said socket thereby to form said stable connection.

7. A method of press-fitting two aluminum parts together to form a stable connection of unquestionable integrity and characterized by high pull-apart resistance suitable for operation at high speeds and comprising the steps of forming a socket on one of the parts, forming a stern on the other of the parts with an interference dimension with respect to the socket, immersion plating one of said stem and said socket in a chemical immersion plating solution of a soft metal such as tin to a soft metal thickness of substantially at least 0.00004 inch; pressing said stem into said socket without galling either said stem or said socket while controlling the press of said stem with respect to said socket to an amount at which the final inertial load between said stern and said socket will not cause deformation of said socket thereby to form said stable connection.

8. A radial compressor of the type including a cylinder housing having a radial bore including a cylinder wall therein and a removable cylinder head closing an open end of the housing; a drive shaft assembly insertable into said cylinder housing through said open end, said assembly incuding a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, and a yoke having bearing surfaces in engagement with opposite surfaces of said slider block; a piston assembly connected to said yoke so that the piston extends from the end of said yoke, said piston assembly being press-fit connected to said yoke through said bore providing a stable connection therebetween of unquestionable integrity and characterized by high pull-apart resistance during compressor operation, said press-fit connection comprising an aluminum stem and shoulder on said piston assembly and an aluminum socket on said yoke, the diameter of said stem before the press-fit connection being larger than the diameter of said socket and one of said stem or said socket having a thin surface plating of soft metal taken from the group consisting of tin and cadmium and having the characteristic to form a gall-preventing lubricant as the stem is pressed into the socket through said bore without deformation between said socket and said shoulder therby to facilitate the press-fit connection, said piston including a piston-skirt having a sealing ring disposed around the skirt providing a bearing surface said piston skirt and the cylinder wall of said bore, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providng substantial area of engagment engagement metal-tometal contact between said piston skirt and said cylinder wall, and valve means to direct fluids to and from the bore.

9. A unitary machine subassembly of unquestionable integrity subject to cyclical reciprocating forces along an axis and concurrent lateral cocking forces of like frequency, the assembly being characterized by high pull-apart resistance after prolonged cyclical duty under the combined axial and cocking forces, the assembly comprising a female part of aluminum having a socket of predetermined size,

a male part of aluminum having a mating stem insertably received in said socket without causing deformation of said socket by the final inertial load of the insertion, one of said stem and said socket having a gall-preventing coating taken from the group consisting of tin and cadmium of effective thickness of at least 0.00004 inch and, prior to insertion, interfering with the predetermined size of said socket in a substantial friction-creating amount less than the amount to cause galling during insertion and greater than the amount to which the assembly is exposed during cyclical reciprocation thereof whereby the assembly maintains its integrity by resisting cyclical pull-apart force in coaction with the cyclical cocking force, 

1. A radial compressor of the type including a cast cylinder housing having cross bores including cylinder walls therein along axes normal to each other, said housing having one closed end and a removable cylinder head closing its other end; a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, a pair of yokes disposed normal to each other and having bearing surfaces in engagement with normal surfaces of said slider block, pairs of piston assemblies connected to each yoke so that the pistons extend oppositely from each other from said yokes, said piston assemblies being press-fit connected to said yokes providing a stable connection therebetween, said press-fit connection comprising an aluminum stem on each of said piston assemblies and an aluminum socket on its respective yoke, the diameter of said stem before the press-fit connection being larger than the diameter of said socket and one of said stem or said socket having a thin surface plating of soft metal having the characteristic to form a lubricant as the stem is pressed into the socket thereby to facilitate the press-fit connection, said pistons including a piston-skirt having a sealing ring disposed around the skirt providing a bearing surface between said piston skirt and the cylinder walls of said cross bores, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providing substantial area of engagement preventing metal-to-metal contact between said piston skirts and said cylinder walls, and valve means to direct fluids to and from the cross bores.
 1. A RADIAL COMPRESSOR OF THE TYPE INCLUDING A CAST CYLINDER HOUSING HAVING CROSS BORES INCLUDING CYLINDER WALLS THEREIN ALONG AXES NORMAL TO EACH OTHER, SAID HOUSING HAVING ONE CLOSED END AND A REMOVABLE CYLINDER HEAD CLOSING ITS OTHER END; A DRIVE SHAFT ROTABLY MOUNTED IN SAKD HOUSING AND SAID CYLINDER HEAD, AN ECCENTRIC ON SAID DRIVE SHAFT, A SLIDER BLOCK ROTABLY MOUNTED ON SAID ECCENTRIC, A PAIR OF YOKES DISPOSED NORMAL TO EACH OTHER AND HAVING BEARING SURFACES IN ENGAGEMENT WITH NORMAL SURFACES OF SAID SLIDER BLOCK, PAIRS OF PISTON ASSEMBLIES CONNECTED TO EACH YOKE SO THAT THE PISTONS EXTEND OPPOSITELY FROM EACH OTHER FROM SAID YOKES, SAID PISTON ASSEMBLIES BEING PRESS-FIT CONNECTED TO SAID YOKES PROVIDING A STABLE CONNECTION THEREBETWEEN, SAID PRESS-FIT CONNECTION COMPRISING AN ALUMINUM STEM ON EACH OF SAID PISTON ASSEMBLIES AND AN ALUMINUM SOCKET ON ITS RESPECTIVE YOKE, THE DIAMETER OF SAID STEM BEFORE THE PRESS-FIT CONNECTION BEING LARGER THAN THE DIAMETER OF SAID SOCKET AND ONE OF SAID STEM OR SAID SOCKET HAVING A THOM SURFACE PLATING OF SOFT METAL HAVING THE CHARACTERISTIC TO FORM A LUBRICANT AS THE STEM IS PRESSED INTO THE SOCKET THEREBY TO FACILITATE THE PRESS-FIT CONNECTION, SAID PISTONS INCLUDING A PISTON-SKIRT HAVING A SEALING RING DISPOSED AROUND THE SKIRT PROVIDING A BEARING SURFACE BETWEEN SAID PISTON SKIRT AND THE CYLINDER WALLS OF SAID CROSS BORES, THE BEARING SURFACE OF SAID SEALING RING BEING SUFFICIENTLY WIDE TO ACCOMPLISH A FLUID TIGHT SEAL WHILE PROVIDING SUBSTANTIAL AREA OF ENGAGEMENT PREVENTING METAL-TO-METAL CONTACT BETWEEN SAID PISTON SKIRTS AND SAID CYLINDER WALLS, AND VLAVE MEANS TO DIRECT FLUIDS TO AND FROM THE CROSS BORES.
 2. The radial compressor of claim 1 wherein the soft metal is taken from the group consisting of tin and cadmium and the thin plating is at least 0.00004 inch thick.
 3. A radial compressor of the type including a cast cylinder housing having cross bores including cylinder walls therein along axes normal to each other, said housing having one closed end and a removable cylinder head closing its other end; a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, a pair of aluminum yokes disposed normal to each other and having bearing surfaces in engagement with normal surfaces of said slider block, pairs of piston assemblies connected to each yoke so that aluminum pistons extend oppositely from each other from said yokes, said piston assemblies being press-fit connected to said yokes providing a stable connection therebetween of unquestionable integrity and characterized by high pull-apart resistance during compressor operation, said press-fit connection comprising a stem on each of said pistons and a socket on its respective yoke, the diameter of said stem before the press-fit connection being large than the diameter of said socket and said stem having a thin plating taken from the group consisting of tin and cadmium to form a gall-preventing lubricant for a press-fit connection without deformation of said socket, said pistons including a piston skirt including an annular groove therein extending a substantial width along said skirt, a sealing ring disposed in said groove providing a bearing surface between said piston skirt and the cylinder walls of said cross bores, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providing substantial area of engagement preventing metal-to-metal contact between said piston skirts and said cylinder walls, and valve means To direct fluids to and from the cross bores.
 4. A radial compressor of the type including a cast cylinder housing having cross bores including cylinder walls therein along axes normal to each other, said housing having one closed end and a removable cylinder head closing its other end; a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, a pair of aluminum yokes disposed normal to each other and having bearing surfaces in engagement with normal surfaces of said slider block, pairs of piston assemblies connected to each yoke so that aluminum pistons extend oppositely from each other from said yokes, said piston assemblies being press-fit connected to said yokes providing a stable connection therebetween of unquestionable integrity and characterized by high pull-apart resistance during compressor operation, said press-fit connection comprising a stem on each of said pistons and a socket on its respective yoke, the diameter of said stem before the press-fit connection being larger than the diameter of said socket and the socket and bearing surfaces of said respective yoke having a thin plating taken from the group consisting of tin and cadmium as a gall-preventing lubricant for a press-fit connection without deformation and for the engagement of the bearing surfaces with said slider block, said pistons including a piston skirt including an annular groove therein extending a substantial width along said skirt, a sealing ring disposed in said groove providing a bearing surface between said piston skirt and the cylinder walls of said cross bores, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providing substantial area of engagement preventing metal-to-metal contact between said piston skirts and said cylinder walls, and valve means to direct fluids to and from the cross bores.
 5. A method of press-fitting an aluminum compressor piston to an aluminum compressor yoke to form a stable connection suitable for reciprocation at high speeds along an axis and subject to concurrent lateral cocking forces of like frequency and comprising the steps of forming a socket on the aluminum yoke, forming a stem on the aluminum piston with an interference dimension with respect to the socket, immersion plating one of said stem and said socket in a sodium stannate chemical immersion plating solution to a tin thickness of substantially at least 0.00004 inch; and pressing said stem into said socket wihtout galling either said stem or said socket while controlling the press of said stem with respect to said yoke to an amount at which the final inertial load between said stem and said yoke will not cause deformation of said socket, thereby to form said stable connection.
 5. A METHOD OF PRESS-FITTING AN ALUMINUM COMPRESSOR PISTON TO AN ALUMINUM COMPRESSOR YOKE TO FORM A STABLE CONNECTION SUITABLE FOR RECIPROCATION AT HOGH SPEEDS ALONG AN AXIS AND SUBJECT TO CONCURRENT LATERAL COCKING FORCES OF LIKE FREQUENCY AND COMPRISING THE STEPS OF FROMING A SOCKET ON THE ALUMINUM YOKE, FORMING A STEM ON THE ALUMINUM PISTON WITH AN INTERFERENCE DIMENSION WITH RESPECT TO THE SOCKET, IMMERSION PLATING ONE OF SAID STEM AND SAID SOCKET IN A SODKIM STANNATE CHEMICAL IMMERSION PLATING SOLUTION TO A TIN THICKNESS OF SUBSTANTIALLY AT LEAST 0.00004 INCH; AND PRESSING SAID STEN INTO SAID SOCKET WITHOUT GALLING EITHER SAID STEM OR SAID SOCKET WHILE CONTROLLING THE PRESS OF SAID STEM WITH RESPECT TO SAID YOKE TO AN AMOUNT AT WHICH THE FINAL INTERTIAL LOAD BETWEEN SAID STEM AND SAID YOKE WILL NOT CAUSE DEFORMATION SAID SOCKET, THEREBY TO FORM SAID STABLE CONNECTION.
 6. A method of press-fitting two aluminum parts together to form a stable connection of unquestionable integrity and characterized by high pull-apart resistance suitable for operation at high speeds and comprising the steps of forming a socket on one of the parts, forming a stem on the other of the parts with an interference dimension with respect to the socket, immersion plating said socket in a sodium stannate chemical immersion plating solution to a tin thickness of substantially at least 0.00004 inch; and pressing said stem into said socket without galling either said stem or said socket while controlling the press of said stem with respect to said socket to an amount at which the final inertial load between said stem and said socket will not cause deformation of said socket thereby to form said stable connection.
 7. A method of press-fitting two aluminum parts together to form a stable connection of unquestionable integrity and characterized by high pull-apart resistance suitable for operation at high speeds and comprising the steps of forming a socket on one of the parts, forming a stem on the other of the parts with an interference dimension with respecT to the socket, immersion plating one of said stem and said socket in a chemical immersion plating solution of a soft metal such as tin to a soft metal thickness of substantially at least 0.00004 inch; pressing said stem into said socket without galling either said stem or said socket while controlling the press of said stem with respect to said socket to an amount at which the final inertial load between said stem and said socket will not cause deformation of said socket thereby to form said stable connection.
 8. A radial compressor of the type including a cylinder housing having a radial bore including a cylinder wall therein and a removable cylinder head closing an open end of the housing; a drive shaft assembly insertable into said cylinder housing through said open end, said assembly incuding a drive shaft rotatably mounted in said housing and said cylinder head, an eccentric on said drive shaft, a slider block rotatably mounted on said eccentric, and a yoke having bearing surfaces in engagement with opposite surfaces of said slider block; a piston assembly connected to said yoke so that the piston extends from the end of said yoke, said piston assembly being press-fit connected to said yoke through said bore providing a stable connection therebetween of unquestionable integrity and characterized by high pull-apart resistance during compressor operation, said press-fit connection comprising an aluminum stem and shoulder on said piston assembly and an aluminum socket on said yoke, the diameter of said stem before the press-fit connection being larger than the diameter of said socket and one of said stem or said socket having a thin surface plating of soft metal taken from the group consisting of tin and cadmium and having the characteristic to form a gall-preventing lubricant as the stem is pressed into the socket through said bore without deformation between said socket and said shoulder therby to facilitate the press-fit connection, said piston including a piston-skirt having a sealing ring disposed around the skirt providing a bearing surface said piston skirt and the cylinder wall of said bore, the bearing surface of said sealing ring being sufficiently wide to accomplish a fluid tight seal while providng substantial area of engagment engagement metal-to-metal contact between said piston skirt and said cylinder wall, and valve means to direct fluids to and from the bore. 